Development of a Single Monarch Butterfly from Egg to Adult
In recent years, I have been raising monarch
butterflies from eggs laid on milkweed plants in my yard in indoor
cages. The reason for doing this, rather than observing them on the
plants where the eggs were laid, is that few if any, would survive to
adulthood without intervention. Although monarch butterflies are
toxic and seldom eaten by birds, there are numerous predators and
parasites that prey on the eggs, larvae, and pupae.
I have been doing this for 3 summers, now. In the first summer, I successfully raised about 10, and last year I raised 12. However, 2020 seems like a poor year for monarchs in the Rochester, NY area. This year, I only succeeded in raising a single egg to adulthood and only found 2 in my yard, even though there is plenty of milkweed. However, raising only one individual gave me the opportunity to photograph the same one at least once a day and closely follow its development.
The metamorphosis of a caterpillar to a pupa and adult still seems somewhat miraculous to me and is still a source of wonder. It seems a bit like dying and being reborn as something quite different. A lowly worm-like caterpillar is transformed into a beautiful and glorious butterfly. Indeed, I wonder if ancient Buddhists observed this transformation, and it gave rise to the concepts of reincarnation and transmigration of souls. To be sure, monarch caterpillars only give rise to monarch butterflies, and not to other kinds of butterflies or completely different creatures.
Raising a single caterpillar gave me the opportunity to observe it in considerable detail and make use of my photographic and quantitative skills to analyze its growth as shown in the chart which is the final image in this gallery.
Some Notes on Methodology
Butterfly rearing: I watched the milkweeds in my yard for visits by a female monarch butterfly. After one was seen, I inspected the plants for eggs. If one or more eggs was found, I brought the leaf into the house and inserted the stem into the perforated lid of a plastic dish containing water. The dishes with leaves were housed in a commercial butterfly house with mesh sides and top. The leaves were replaced with fresh ones as needed without touching the caterpillar.
Photography: A Canon EOS 60D digital camera equipped with a Canon EF-S 60 mm 1:1 macro lens and a Sigma EM-140DG ring flash was used in manual exposure mode at f16 and 1/250th second. This exposure mode gives adequate depth of focus while avoiding motion blur and minimizing unwanted effects of ambient light. The camera automatically records the focal distance of the subject. This is a useful feature that enables me to estimate the sizes of objects in the photos. The lens is calibrated with a scale that shows the magnification factor from 1:1 at the minimum focusing distance of 0.2 m to 1:5 at a distance of 0.44 m. There is a linear relationship between the focusing distance and the magnification factor over this range.
Calculations and data analysis: Dimensions in pixels were measured with ImageJ and converted to mm by using the magnification factor calculated from the focal distance and the pixels/mm of the camera sensor. Volumes were calculated from the length and width of the caterpillars using the formula for the volume of a cylinder. Calculations, graphing, and curve fitting were done with the Apple Numbers '09 spreadsheet on a Macintosh computer.